1. Field of the Invention
The present invention relates to a spread code allocation method and a base station in a CDMA cellular in which transmission signals directed from a base station to a plurality of mobile stations are spread and transmitted using spreading codes.
2. Description of the Related Art
On the downlink of a CDMA cellular, transmission signals directed to a plurality of mobile stations are spread and multiplexed by different spreading codes at a base station and then transmitted in a group. On the downlink, transmission signals can be synchronized with each other with ease because signals directed to the plurality of mobile stations are transmitted from the base station in a group. It is therefore possible to suppress interference between transmission signals by spreading each transmission signal by an orthogonal synchronization code.
On the other hand, since signals on a downlink from other base stations are hard to synchronize with each other, employed as a means for suppressing interference from these base stations is a method of multiplying an orthogonal synchronization code by a scramble code inherent to a base station. Used here as a scramble code is a gold code or the like.
Structure of a base station transmission device using the present method is shown in FIG. 12. In a spreading addition device spu shown in FIG. 12, a transmission signal Si (i: mobile station number) directed to a mobile station and a orthogonal code Co, i are multiplied at a multiplier 001, the multiplied output signal is added at an adder 002 and the output signal of the adder 002 is multiplied by a scramble code Cs at the multiplier 005 to output a composite spreading signal sd. The composite spreading signal sd is modulated at a modulation device 003 and then transmitted from an antenna 004 to each mobile station.
Use of the above described diffusion method shown in FIG. 12 by all the base stations results in randomizing interference of a downlink from other base station received at a mobile station, so that together with error correcting codes, interference can be suppressed.
Since code consumption is accelerated in a CDMA cellular system for the following reasons, there often occur a lack of orthogonal codes.
First, when Soft Hand-Over (SHO) is applied, because a plurality of base stations transmit the same signal for one mobile station, more codes are consumed as compared with those consumed when no SHO is applied. In addition, since in a voiceless period, signal transmission is temporarily stopped to reduce interference, thereby accommodating more mobile stations, more codes are consumed than those in the transmission of information sources being on all the time. Moreover, since code consumption is changed also with propagation environments and in a case where many shielding objects exist on a propagation path or other case, interference of other cells is well suppressed to increase a capacity, code consumption at a base station is increased.
For the foregoing reasons, when more mobile stations are connected to a base station than the number of orthogonal codes, a lack of spreading codes is coped with by the use of a plurality of scramble codes at one base station.
FIG. 13 is a diagram showing a structure of a base station transmission device in a case where a number k of scramble codes are used. At a spreading addition device spu-j (j: scramble code number), a transmission signal Si, h (h: orthogonal code number), a orthogonal code Co, h and a scramble code Cs, j are applied to output a composite spreading signal sd-j. nj in FIG. 13 denotes the number of uses of a orthogonal code generated by a j-th scramble code.
After subjected to addition at an adder 002, the output sd-j of the composite spreading signal is modulated at a modulation device 003 and then output to each mobile station through an antenna 004.
In a case where base station simultaneously uses a plurality of scramble codes as shown in FIG. 13, and the transmission signals have different required qualities such as transmission rate, required bit error rate, and different reception qualities depending on where a mobile station is within a cell, the amount of interference, etc. The degree of interference that each transmission signal has varies due to a difference in allocated scramble code thereby causing inequality in communication quality and deterioration due to interference.